A light emitting (LED) is a semi-conductive lighting element widely which is used and serves as an indicator light and a display screen. People praise the white light emitting diode (WLED) as the fourth generation light source which substitutes for fluorescent lights and incandescent lights. Theoretically, although the life span of LEDs is 100,000 hours, however, the life span of a white light emitting diode (WLED) in the spot market is much less than the theoretical time of 100,000 hours.
FIG. 1 is a graph of a test profile of luminance decay for a conventional white light emitting diode (WLED) wherein the WLED is tested at a constant current 25 mA in the room temperature 25° C. during a continuous duration of 2520 hours. The horizontal axis represents time and the vertical axis represents the luminous flux maintenance percentage. According to the graph, the rate of luminance decay at 168th hour is 13%, the rate of luminance decay at 336th hour is 20%, the rate of luminance decay at 504th hour is 25%, the rate of luminance decay at 648th hour is 31%, the rate of luminance decay at 1128th hour is 49%, and the rate of luminance decay at 2520th hour is 80%. To further verify the rate of luminance decay, the WLED is tested at a constant current 40 mA in room temperature 25° C. during a continuous duration of 1032 hours, as shown in FIG. 2. The rates of luminance decay at 168th, 336th, 504th, and 1032th hours are 34%, 45%, 68%, and 88%, respectively.
In view of luminance decay of WLED, many manufactures attempt to make efforts to reduce luminance decay. For example, as shown in FIG. 3, it is a graph of a test profile of luminance decay for a conventional WLED, mode number of “NSPW500CS”, manufactured by “NICHIA” Corporation wherein the WLED is tested at a constant current 25 mA in room temperature 25° C. during a continuous duration of 2520 hours. According to the graph, the rate of luminance decay at 168th hour is 3%, the rate of luminance decay at 336th hour is 5%, the rate of luminance decay at 504th hour is 8%, the rate of luminance decay at 648th hour is 10%, the rate of luminance decay at 1128th hour is 12%, and the rate of luminance decay at 2520th hour is 24%. To further verify the rate of luminance decay, the WLED is tested at a constant current 40 mA in room temperature 25° C. during a continuous duration of 1032 hours, as shown in FIG. 4. The rates of luminance decay at 168th, 336th, 504th, and 1032th hours are 10%, 13%, 14%, and 24%, respectively.
FIG. 5 is a graph of a test profile of luminance decay for a conventional WLED, mode number of “LC503TWN1-15Q-A1”, manufactured by “COTCO” Corporation wherein the WLED is tested at a constant current 25 mA in room temperature 25° C. during a continuous duration of 2520 hours. According to the graph, the increment of luminous flux maintenance percentage at 168th hour is 4%, the luminous flux maintenance percentage at 336th hour is still 104%, the luminous flux maintenance percentage at 504th hour is dropped as 103%, the luminous flux maintenance percentage at 648th hour is continuously dropped as 101%, the rate of luminance decay at 1128th hour is 3%, and the rate of luminance decay at 2520th hour is up to 9%. To further verify the rate of luminance decay, the WLED is tested at a constant current 40 mA in room temperature 25° C. during a continuous duration of 1032 hours, as shown in FIG. 6. The increment of luminous flux maintenance percentage at 168th hour is 4%, the rate of luminance decay at 336th hour is 2%, the rate of luminance decay at 504th hour is 7%, and the rate of luminance decay at 1032th hour is up to 17%.
According to the above-mentioned descriptions, the luminance decay of the WLEDs manufactured by “NICHIA” and “COTCO” is reduced. However, the prices of WLEDs are much too high, thereby increasing the manufacturing cost of the WLEDs and restricting their application fields.